Numerous of the combustion processes incident to organic waste destruction generate as well as an undesired product, effluent gases having an unacceptable NO.sub.x content. More specifically, the high temperatures incident to the thermal destruction results in the fixation of some oxides of nitrogen. These compounds are found in the effluent gases mainly as nitric oxide (NO) with lesser amounts of nitrogen dioxide (NO.sub.2) and only traces of other oxides. Since nitric oxide (NO) continues to oxidize to nitrogen dioxide (NO.sub.2) in the air at ordinary temperatures, there is no way to predict with accuracy the amounts of each separately in vented gases at a given time. Thus, the total amount of nitric oxide (NO) plus nitrogen dioxide (NO.sub.2) in a sample is determined and referred to as "oxides of nitrogen" (NO.sub.x).
Such NO.sub.x emissions from stack gases, engine exhausts, etc., through atmospheric reactions, produce "smog"0 that stings eyes and may cause or contribute to acid rain. Other deleterious effects both to health and to structures are believed to be caused directly or indirectly by these NO.sub.x emissions. For these reasons, the content of oxides of nitrogen present in gases vented to the atmosphere has been subject to increasingly stringent limits via regulations promulgated by various state and federal agencies.
The Clean Air Act Amendments of 1990 (CAAA) will have a profound impact on sources that emit air toxics found in the new law's list of hazardous air pollutants. Completely restructuring the existing law, Congress has granted the U.S. Environmental Protection Agency (EPA) the authority to regulate hazardous emissions by categories of industry rather than individual pollutants and to develop control standards based on "maximum achievable control technology" (MACT) for each category. The geographic coverage of the control programs has also been greatly expanded. Using geographic designations, the new law seeks to regulate sources of nitrogen oxides (NO.sub.x), volatile organic compounds (VOC), carbon monoxide (CO), and particulate matter (PM) based on five discrete categories for areas that do not meet National Ambient Air Quality Standards (NAAQS) for ozone.
A common prior art approach to destruct or prevent the formation of oxides of nitrogen in the treatment of exhaust streams that contain nitrogen-based compounds is to employ either thermal or catalytic processes. Thermal processes are effective in destruction of these compounds, but result in unacceptable levels of NO.sub.x. The provisions of the cited new law present a real dilemma for facilities that have hazardous air pollutants in ozone non-attainment areas. If thermal treatment is applied as MACT, the result may actually be an increase in the level of NO.sub.x, which is at odds with other provisions of the law requiring reductions in NO.sub.x and CO emissions.
The foregoing dilemma is in part illustrated by an existing situation, wherein a chemical manufacturing corporation operates a staged, thermal incineration unit. The system is based on a process disclosed in U.S. Pat. No. 3,873,671. The system utilizes "staged" combustion to destroy air toxics from two process vent streams in the manufacturing plant. The system, however, has not achieved satisfactory destruction efficiencies for hazardous air pollutants and emitted 250-450 ppmv NO.sub.x. This staged combustion concept (see prior art FIG. 1) utilizes a first stage that is operated with an excess of fuel relative to available oxygen (O.sub.2) to create a high temperature reaction zone in which hazardous air pollutants are thermally dissociated and NO.sub.x is reduced by the excess "combustibles", primarily carbon monoxide (CO) and hydrogen (H.sub.2). This is followed by a cooling step in which recycled flue gas from the stack is mixed with the effluent from the first stage to reduce the temperature to 1400.degree.-1600.degree. F., a level which minimizes thermal NO.sub.x formation. After the cooling step, make-up air is added to provide the second combustion stage which is operated under "oxidizing" conditions at 1800.degree.-2000.degree. F. to convert the excess "combustibles" to normal products of combustion, carbon dioxide (CO.sub.2), and water vapor.
This approach was introduced in the early 1970s and demonstrated that NO.sub.x reductions to levels in the range of 200-250 ppmv could be readily achieved. Although this two-stage method has been successfully applied to a number of applications requiring NO.sub.x reduction, it has several drawbacks. One is its limitation on the destruction of air toxics and CO that can be achieved. The other is the higher levels of excess fuel required to achieve adequate NO.sub.x reduction.
The vapor stream was being vented to the two-stage incinerator. The composition and flow rate are shown in Table I. There is sufficient oxygen in the stream to provide stable burning in the vortex burner of the incinerator. The unit required about 7.0 million Btu per hour of auxiliary fuel to react with the oxygen (O.sub.2), hydrogen cyanide (HCN), CO, and NO.sub.x in the vent stream. In order to maximize NO.sub.x reduction, a minimum of 50% excess fuel is fired in the burner, bringing the total firing rate up to 10.50 Btu/hr. This results in a combustibles (CO and H.sub.2) concentration of about 12.5% which is the driving force for NO.sub.x reduction.
TABLE I ______________________________________ Vent Stream Flow and Composition Component Stream (Vol. %) ______________________________________ N.sub.2 58 O.sub.2 16 CO.sub.2 9 CO 2 H.sub.2 O 6 NO 5 NO.sub.2 4 Flow (SCFM) 1206 ______________________________________
This high level of combustibles is advantageous for NO.sub.x reduction, but it can create some problems. First, the formation of CO and H.sub.2 by the dissociation of CH.sub.4 is an endothermic reaction and tends to lower the operating temperature in the reaction zone. Also, since the rate of reaction of the combustibles with the NO.sub.x is a function of temperature (as the temperature is lowered the reaction rate decreases), the lower temperature resulting from the dissociation of excess fuel somewhat offsets the benefit of the higher concentration of the reducing reactants (CO and H.sub.2).
Second, the combustibles that leave the reduction section must be converted to products of combustion in the oxidation section. As the level of combustibles increases, more air and recycle to provide "burn out" of the combustibles and control of the oxidation chamber temperature are required. These high flow rates can result in a low residence time in the oxidation zone that is not sufficient to achieve the required level of CO oxidation.
The third potential problem with high excess fuel operation is the tendency to form HCN and NH.sub.3 under reducing conditions. As the temperature is lowered and the excess fuel is increased, formation of these by-products is favored. This, coupled with low residence time in the oxidation section prevents adequate destruction of HCN and CO. The plant had experienced both high levels of CO (1300-1700 ppmv) and appreciable quantities of HCN (50-100 ppmv) in the stack gases which vent to the atmosphere. Although the high levels of HCN have been attributed to the fact that HCN (.about.200 ppmv) enters the staged, thermal incinerator in the vent stream and is probably not generated at these levels in the reduction chamber, the same conditions which lead to HCN formation also will lead to minimum HCN destruction. The high levels of CO in the stack are required to maintain NO.sub.x levels in the stack below 400 ppmv.
The chemical manufacturer is in an ozone non-attainment area and wanted to expand production. To process a permit for this expansion, NO.sub.x offsets, in addition to lowering the levels of CO and HCN in the vent from the two-stage incinerator, are being sought. Quantitatively, the chemical manufacturer wanted to achieve a 98% reduction in HCN, a 95% reduction in CO, and a 90% reduction in NO.sub.x.
In the present applicant's U.S. Pat. Nos. 5,022,226 and 5,224,334, cogeneration processor and systems are disclosed which are particularly useful where internal combustion engines are employed as the primary power source, and which ensure extremely low NO.sub.x content in the final exhaust gases vented to ambient. Thus, in U.S. Pat. No. 5,022,226 patent, a cogeneration system is provided wherein fuel and oxygen are provided to an internal combustion engine connected to drive an electric generator, to thereby generate electricity. An exhaust stream is recovered from the engine at a temperature of about 500.degree. to 1000.degree. F. which includes from about 6 to 15 percent oxygen. Sufficient fuel is added to the exhaust stream to create a fuel-rich mixture, the quantity of fuel being sufficient to react with the available oxygen and reduce the NO.sub.x in the exhaust stream. The fuel-enriched stream is then provided to a thermal reactor means for reacting the fuel, NO.sub.x and available oxygen, to provide a heated oxygen-depleted stream. The oxygen-depleted stream is cooled in a heat exchanger. Prior to being passed over a catalyst bed under overall reducing conditions, conversion oxygen is added to the cooled stream. Such oxygen can be provided directly (i.e. as air), but preferably can be provided by bypassing part of the exhaust stream from the engine. The quantity of conversion oxygen is stoichiometrically in excess of the amount of NO.sub.x but less (stoichiometrically) than the amount of combustibles, in consequence of which NO in the stream is oxidized to NO.sub.2 at the forward end of the bed, after which the NO.sub.2 is reduced in the remainder of the bed by the excess combustibles. Air is added to the resulting stream from the catalytic bed to produce a cooled stream having a stoichiometric excess of oxygen, and the stream is passed over an oxidizing catalyst bed to oxidize remaining excess combustibles. The resultant stream, vastly reduced in NO.sub.x content can then be provided for venting. By means of the U.S. Pat. No. 5,022,226 invention, the NO.sub.x content can be reduced to less than 25 ppmv--often below 15 ppmv, while CO levels are also brought to well below 50 ppmv.
Pursuant to the U.S. Pat. No. 5,224,334 patent, it was found that the limiting factor for overall NO.sub.x reduction in the method and system of U.S. Pat. No. 5,022,226, is not as had previously been believed, the destruction of NO.sub.x in the reduction catalyst step. Essentially all of the NO.sub.x is reduced in this step, but apparently by-product ammonia is formed and is thereupon oxidized across the oxidation catalyst to reform NO.sub.x. If, as taught in the U.S. Pat. No. 5,022,226, the oxidation catalyst step is operated at substantially the same temperature as that for the reduction catalyst (about 750.degree.-1250.degree. F.), 60-80% of the ammonia formed in the reduction step will be oxidized to reform NO.sub.x. Pursuant to U.S. Pat. No. 5,224,334, however, it was found that by cooling the effluent stream from the reduction catalyst to about 400.degree. to 600.degree. F., and preferable to around 500.degree. to 550.degree. F. prior to the catalytic oxidation step, the oxidation of ammonia to form NO.sub.x is minimized. NO.sub.x levels are thereby reduced to remarkably low levels, typically below 5 ppmv.